1. Field of the Invention
The present invention relates to improvements in a vehicle height control apparatus, and more particularly to the vehicle height control apparatus in which a vehicle height is adjusted by controlling the pressure in each fluid chamber installed to each suspension.
2. Description of the Prior Art
A variety of vehicle height control apparatuses for vehicles have been proposed and practically used. A typical vehicle height control apparatus is disclosed, for example, in Japanese Patent Provisional Publication No. 63-154413, in which each suspension is installed between a vehicle body and each wheel. The suspension includes a fluid chamber through which a vehicle height is varied. When the vehicle height is varied by this apparatus, right and left fluid chambers at one of the front and rear wheel sides do not communicate with each other, and right while the left fluid chambers at the other side are communicate with each other. Thus, this conventional vehicle height control apparatus controls the vehicle height in the manner of a three points control. Accordingly, the hunting phenomena, which occurs at a final stage of the vehicle height control to overly repeat the fine control, is prevented from occurring. With this apparatus, the vehicle height is raised or lowered by supplying the fluid from the fluid supply source to, the fluid chamber or discharging the fluid from the fluid chamber.
However, when the vehicle is parked on the inclined surface which is inclined in the lateral direction of the vehicle, the weight of the vehicle is applied to a lower side wheel of the vehicle. Accordingly, it is necessary that the pressure of the fluid chamber at the lower side wheel be higher than the pressure of the fluid chamber at the upper side wheel of the vehicle, in order to keep the vehicle position horizontal relative to the inclined surface. Furthermore, when the vehicle is parked on the horizontal surface after the above-discussed operation, the vehicle is inclinedly parked by virtue of the pressure difference between the right and left fluid chambers. Although it is easy to adjust the inclination of the vehicle body in this condition since it is possible to supply the fluid to the fluid chamber and discharge the fluid from the fluid chamber, it is difficult in a parked vehicle to raise the vehicle height by supplying the fluid to the fluid chamber. If the vehicle heights of the right and left sides are greater than the lower limit of a target range, the vehicle height is adjusted by lowering the vehicle height of the higher side even if the vehicle is in a condition that the vehicle height control apparatus cannot operate to raise the vehicle height. However, when the vehicle height of the lower side is smaller than the lower limit of the target range and the vehicle is in the condition that the vehicle height control apparatus cannot operate to raise the vehicle height, the vehicle height control is not carried out, and the inclination of the vehicle is maintained in a bad condition.